The addition of valves made it possible to play low in the harmonic series of the instrument and still have a complete selection of notes. Prior to the invention of valves, brass instruments were limited to notes in the harmonic series, and were thus generally played very high with respect to their fundamental pitch. Harmonics starting three octaves above the fundamental pitch are about a whole step apart, making a useful variety of notes possible.The ophicleide used a bowl-shaped brass instrument mouthpiece but employed keys and tone holes similar to those of a modern saxophone. Another forerunner to the tuba was the serpent, a bass instrument that was shaped in a wavy form to make the tone holes accessible to the player. Tone holes changed the pitch by providing an intentional leak in the bugle of the instrument. While this changed the pitch, it also had a pronounced effect on the timbre. By using valves to adjust the length of the bugle the tuba produced a smoother tone that eventually led to its popularity.